The present invention relates to mobile phones or radio apparatus for receiving spread spectrum signals, such as code division multiple access (CDMA) signals, with which time synchronization is achieved.
Radiocommunication systems involve the transmission of information over an air interface, for example by modulating a carrier frequency with that information. Upon reception, a receiver attempts to accurately extract the information from the received signal by performing an appropriate demodulation technique. However, in order to demodulate a received signal, it is first necessary to synchronize timing between the transmitter and the receiver. Different levels of synchronization may be required depending upon the design of the radiocommunication system. For example, in most systems clocking differences between the transmitter and the receiver create differences in timing at a bit level. Moreover, in some radiocommunication systems, information is transmitted in bursts, sometimes referred to as xe2x80x9cframesxe2x80x9d, which represent chunks of information that are independently detected and decoded. In these types of systems it is also desirable to locate the beginning of a frame, so that information relevant to a particular receiver is isolated and demodulated. Likewise, some systems (e.g., time division multiple access or TDMA systems) further subdivide frames into timeslots to create channels that are time multiplexed with one another. In these systems it is further desirable to synchronize the receiver to the beginning of each timeslot.
Some systems provide channelization using a spread spectrum technique known as code division multiple access (CDMA). In CDMA systems, the information data stream to be transmitted is first coded or spread using a unique spreading code and then combined with a long PN-sequence or a shorter scrambling-sequence. In the latter case, the scrambling-sequences are planned from cell to cell so that neighboring cells use different scrambling-sequences or scrambling-masks. The information data stream and the PN-sequence or the scrambling sequence can have the same or different bit rates. The multiplication of the information data stream with the unique spreading code and long PN-sequence results in an output stream of chips. Thus, in CDMA systems, it is also desirable to synchronize the receiver to the chip boundaries.
To further understand the synchronization tasks associated with signal processing in a CDMA radiocommunication system, consider the following example. FIG. 1 illustrates the use of base stations to transmit radio waves to mobile users (mobile stations) in a cellular system. In a CDMA system, base station 10 can transmit signals to mobile stations 14 and 15 as a single (composite) signal. The signal directed to mobile station 14 is typically coded with a short code that is orthogonal or mostly orthogonal to a short code that is used to code the signal directed to mobile station 15. These signals are then spread with a second code that is sometimes referred to as a long code, associated with base station 10. The sum of the two coded and spread signals is then transmitted by base station 10.
When mobile station 14 receives the composite signal, mobile station 14 multiplies the spread signal with the long code and the short code to recreate the signal directed to mobile station 14 and the signal directed to mobile station 15 is suppressed as interference noise. Similarly, mobile station 15 multiplies the spread signal with the long code and the short code assigned to mobile station 15 to recreate the signal directed to mobile station 15 and the signal directed to mobile station 14 is suppressed as interference noise. The receivers associated with mobile stations 14 and 15 must have acquired the various levels of synchronization to the received signal which were described above, in addition to learning or knowing the applicable long and short codes, in order to implement despreading, demodulation and decoding of the information residing in that signal. Many different techniques have been developed in order to acquire synchronization at each of the various levels. For frame synchronization these techniques typically depend, in large degree, on the frame structure and the manner in which overhead or control information is conveyed to the mobile station. Overhead information is usually provided on one or more broadcast control channels which are transmitted by base stations using known channels to which mobile stations can quickly lock onto and receive the overhead information including, among other things, information used to acquire frame synchronization with that base station. Those skilled in the art will appreciate that many radiocommunication systems have unsynchronized base stations, i.e., base stations which do not share a common timing reference signal. Accordingly, frame synchronization is a process which needs to be performed, for example, at start-up (i.e., when a mobile is powered on), as a mobile station moves from cell to cell and when measuring on channels associated with neighboring cells as part of cell reselection procedures (e.g., to confirm that a mobile station is listening to a xe2x80x9cbestxe2x80x9d serving base station).
Of course, as with most signal processing tasks performed by the receiver, reducing the delay associated with synchronization is important in improving the receiver""s performance. Many types of communication services, in particular speech communication, are relatively delay intolerant. Thus, system designers are continuously seeking for ways in which to reduce the amount of time that it takes to perform any given signal processing task, including time synchronization.
A radio receiver employing a synchronization method according to the present invention is able more rapidly to find and acquire synchronisation with a CDMA signal by means of at least a two step process in which, at a first step, a number of candidate synchronisation frequencies or timings are identified, followed by confirming one of the candidates as a correct synchronisation state at a second or final step. According to the present invention, the confirmation step may be performed at the same time as the step of identifying further candidates by processing the same received signal samples in different ways.
In an exemplary implementation, a first correlation means correlates shifts of a stream of received signal samples using a correlation length over which the received signal does not drift significantly in phase, amplitude or timing, thus allowing coherent correlation. Coherent correlations are not in general expected to reach a sufficient signal-to-noise ratio to unambiguously identify with adequate certainty that correct synchronisation has been achieved. Consequently a number of coherent correlations corresponding to like timing postulates are further accumulated non-coherently in a number of bins, each bin corresponding to a timing postulate. Non-coherent accumulation involves adding the magnitudes or square magnitudes of the coherent correlations, where the squared magnitude is equal to the sum of the squares of the real and imaginary parts of the coherent correlation value.
When non-coherent or magnitude accumulation must take place for a prolonged time period in order to identify a likely candidate timing, and the time period is so long that a drift of timing may occur that is of the order of plus or minus one timing bin width or more, the present invention may employ a drift compensation type of accumulation described in U.S. patent application Ser. No. 08/768,975 to Paul W. Dent, filed Dec. 18, 1996, the disclosure of which is hereby incorporated by reference herein.
An exemplary CDMA system according to the present invention, using a 4 megachip per second modulation, searches time bins that are one chip wide, i.e. 0.25US. The receiver time and frequency reference has an initial error of +/xe2x88x9210 parts per million, which results in a drift of one bin per 25 mS. This exemplary CDMA system furthermore transmits a known chip pattern every 0.625 mS for use by the receiver to achieve a first synchronization step. The known chip pattern has a length of 256 chips for example. With 10 ppm frequency error, the maximum coherent correlation length is restricted to about 64 chips. Therefore four, 64-chip coherent correlations are accumulated non-coherently every 0.625 mS to determine a correlation value with the known 256-chip pattern. Approximately 40 of these 0.625 mS correlations may then be accumulated non-coherently, after which the timing may have drifted one chip. To compensate for such drift, after a number of non-coherent accumulations less than 40, for example 16, the results for each bin are accumulated with the best of previous cumulative results lying within +/xe2x88x921 bin, thus allowing a drift of +/xe2x88x92 one bin in 16 while still providing continued accumulation. In the above exemplary system, the number of timing bins is approximately 0.625 mSxc3x974 megachips/sec, that is about 2500 bins of one-chip width.
Drift-compensated accumulation continues according to the present invention until a cumulative results bin reaches a threshold. This indicates that the known signal pattern may have been found at the timing corresponding to the threshold. The timing associated with that bin is then recorded in a list of candidate timings to be further evaluated, and the bin contents reset to zero. Drift compensated accumulation of correlations with the known signal pattern then continues and as further bins reach the first threshold, their associated timings are also entered into the list and their bin contents reset to zero. Eventually, a bin already reset to zero at least once may again reach the threshold value and would then be entered into the list a second time. The list thus contains candidate timings in the order in which they reached the first correlation threshold, including possible repeats of an earlier candidate timing. Drift-compensated accumulation in timing bins can comprise subtracting the minimum value over all bins from all the bins, so as to emphasize differences between the bin values and prevent indefinite growth of numerical values. Detecting that a bin value reaches a threshold can then imply detecting that a bin value has exceeded the other bin values by a threshold.
In the exemplary CDMA system, many signals may be transmitted in the same bandwidth at the same time using different spreading codes, preferably orthogonal codes. In addition to the above-mentioned known signal pattern, which typically uses the same code, regardless of the transmitter, a second signal pattern is transmitted that is selected to be different for different transmitters in an adjacent group of transmitters. The second signal pattern being one of a limited number of, for example, 16 predetermined patterns. A second stage of synchronisation therefore consists of determining if one of the second known signal patterns can be found at a timing shift recorded in the candidate list derived in the first step described above. Because the second known signal pattern is transmitted at the same time and on the same frequency as the first known signal pattern, they are both received overlapping at the receiver and correlations may be performed using the second known symbol pattern by a second correlation means simultaneously with continuing to perform and accumulate correlations with the first known signal pattern using the first correlation means. The invention of Dent and Wang described in U.S. patent application Ser. No. 08/967,444, entitled xe2x80x9cEfficient Correlation Over a Sliding Windowxe2x80x9d (filed Nov. 11, 1997) may, for example, advantageously be employed to combine the first and second correlation means taking advantage of common computations to reduce effort and therefore power consumption.
Correlations are performed with all 16 of the second known signal patterns, but using only that timing shift identified with the first candidate in the list. According to a second aspect of the present invention, correlations with the first known signal pattern may also be used to estimate a frequency error, which is also stored in the list against candidate timings. When using a candidate timing to test for correlations with the 16 second known signal patterns, the frequency error estimate may be used to compensate the received signal for phase drift, allowing a longer coherent second correlation to be performed. When one of the 16 second correlations has reached a second threshold, the timing, frequency error estimate and the second known symbol pattern giving that correlation may be recorded in a second list of candidates to be tested in an optional third synchronization step. The present invention can optionally comprise performing simultaneous correlations with the second known symbol patterns using more than one of the timings from the candidate list in parallel. The results would be accumulated in 16N bins, where N is the number of simultaneously tested candidates. When any one of the 16N correlations reaches the second threshold, the associated second known code, timing and frequency error are transferred to the second list. The second list thus contains second correlations in the order in which their accumulations first reached the second threshold.
In the exemplary system, the second known symbol pattern identifies a group of third known symbol patterns, one of which should be found in a third correlation step. The present invention may be applied again to search simultaneously for a third correlation while continuing first and second correlation accumulations. The third correlation is performed using all known symbol patterns in the group, which once again may, for example, contain 16 members. When a third correlation is identified, the method has thus disambiguated which of 256 different signal waveforms are in use, i.e. the CDMA spreading code used by the transmitter has been narrowed down to one candidate. Using that code, an attempt is then made to decode a broadcast control channel emitted by the transmitter, successful decoding being indicated, for example, by a Cyclic Redundancy Check Code (CRC). This final step confirms that a valid signal has been identified and sychnronisation achieved.
A further uncertainty that is resolved by the method is which of a number of frequency channels the transmitter is using. This is done by applying the method sequentially to all possible frequency channels selected intelligently in a priority order. For example, a frequency channel on which sync was previously found most recently can be tested first. Moreover, sync search on any frequency channel may be abandoned at any stage if the correlation accumulation for that stage fails to reach the threshold for that stage in a given time.